1. Technical Field
The present invention is an imaging method and imaging device that is applied to copiers, laser printers, and facsimiles. In particular, in imaging methods and devices that utilize reverse development, the present invention, by making the separation shift-bias voltage polarity opposite to the transferring voltage, and lkV or more, is so as to prevent image density irregularities due to the influences of transfer ghosts.
2. Description of Related Art
In imaging methods utilizing reverse development, the surface of a photosensitive conductor at first takes a main charge, and an electrostatic latent image corresponding to an original document image by exposing the image portion of the document is formed on the surface of the photosensitive conductor. Next, toner charged at the same polarity as the main charge develops the exposure portions of the electrostatic latent image, to which a developing bias voltage has been applied. Then further, the formed toner image is transferred to a transfer medium utilizing transferring voltage of reverse polarity to the main charge, after which the image is fixed onto the transfer medium. Therein, in the transfer process, the transfer medium, such as paper, contacts the photosensitive conductor and is electrostatically adhered to the photosensitive conductor surface. In a separation process, applying a voltage through the back side of the transfer medium electrostatically neutralizes, strips the charge of, and separates the adhered transfer medium from the photosensitive conductor surface. With such separation processes, methods that apply AC voltage to the transfer medium have been conventionally utilized.
Imaging methods like this are widely used in such imaging devices as digital and analog photocopiers, printers, or ordinary-paper facsimile machines. Therein, imaging methods by reverse development, i.e., imaging methods that develop using toner of the same polarity as the main-charging voltage applied to the photosensitive conductor, in particular are widely used as digital-imaging methods.
FIG. 1 schematically depicts an example of an imaging device that uses reverse development; 1 is a photosensitive conductor that rotates unidirectionally at constant speed, and the photosensitive conductor 1 is a drum base-form on the surface of which a photosensitive layer is formed. Surrounding the photosensitive conductor 1, in its advancing direction--in other words, along its rotational direction--a main-charging unit 2, an exposure unit 3, a reverse-developing unit 4, a transfer unit 5, a separation unit 6 and a charge-stripping device 7 are provided, in that order.
In imaging methods that use reverse development, the transferring voltage applied to the surface of the photosensitive conductor 1 by the transfer unit 5 ordinarily is not applied directly but is applied through a transfer medium 8, and is not applied when the transfer medium 8 does not pass through the transfer unit 5. Nevertheless, on/off timing the transferring voltage is extremely difficult, and transferring voltage gets applied directly to the photosensitive conductor 1, on those portions just before the leading edge and just after the trailing edge of the transfer medium 8. In other words, because the transferring voltage begins to be applied just before the leading edge of the transfer medium 8 reaches the transfer unit 5, and further, the transferring voltage continues to be applied even when the trailing edge of the transfer medium 8 passes the transfer unit 5, at these timings transferring voltage is directly applied to the photosensitive conductor 1.
Further, in reverse development systems the transferring voltage applied to the surface of the photosensitive conductor 1 by the transfer unit 5 is of opposite polarity to that of the main-charging voltage applied with the main-charging unit 2. Therefore, when a transferring voltage greater than the superficial electric potential on the photosensitive conductor 1 is applied, the polarity of those portions on the photosensitive conductor 1 to which voltage is applied directly (below, noted as "direct-applied portions") becomes opposite to that of the superficial electric potential on the photosensitive conductor 1 when given its main charge.
At this point, the photosensitive characteristics of the photosensitive conductor 1 will be explained. As a photosensitive conductor 1, there are, on a conductive base form, the single-layer type, in which electric charge conveying agents, electric charge generating agents, and a binding synthetic polymer are mixed and formed into one layer, and the laminated type, in which a charge conveying layer and a charge generating layer are laminated.
Single-layer type photosensitive bodies, because they contain a positive-hole conveying agent and an electron conveying agent as electric charge conveying agents, have optical sensitivity at positive/negative, dual polarity. Nevertheless, because there is a difference in charge (positive-hole or electron) travel speed between the positive-hole conveying agent and electron conveying agent, generally optical sensitivity during opposite-polarity charging will be remarkably greater than when charged to either polarity. In reverse development systems, to secure satisfactory developing conditions in the reverse-developing unit 4, ordinarily main-charging voltage of polarity for the greater optical sensitivity, and transferring voltage of polarity for the lesser optical sensitivity are utilized.
With laminated type photosensitive bodies on the other hand, whether they are either positive or negative charging is determined by the sequence in which the charge-generating layer and charge-conveying layer are formed, and by the type of charge-conveying agent (electron-conveying agent or positive-hole conveying agent) used for the charge-conveying layer. Herein, there is none of the aforementioned optical sensitivity with respect to charging type and opposite polarity, and stripping charge on the surface of the photosensitive conductor to get rid of dark attenuation is not possible. In reverse-developing systems, main-charging voltages of polarity having optical sensitivity, as well as transferring voltages of polarity not having optical sensitivity are utilized.
As a result, in using either a single-layer or a laminated photosensitive conductor, the superficial electric potential of the direct-applied portions on the photosensitive conductor 1 just after transfer, at polarity opposite to that of the main charge, takes on the polarity of lesser optical sensitivity, or of no optical sensitivity.
In the separation unit 6, a separation voltage is applied to the photosensitive conductor 1 and to the transfer medium 8 after having passed through the transfer unit 5. Ordinarily, because the separation voltage is AC, dual-polarity voltage is applied, which does not result in neutralization of the electric potential of the direct-applied portions on the photosensitive conductor 1. Further, although a shift bias voltage of the same polarity as the main-charging voltage can be superimposed on the separation voltage, wherein the shift bias voltage is lower than the superficial electric potential of the direct-applied portions, the direct-applied portions are as such still of polarity opposite to that of the main-charging voltage.
This makes it all the more unlikely that the superficial electric potential of polarity opposite to that of the remaining main-charging voltage will be removed sufficiently with the charge-stripping beam that is irradiated onto the surface of the photosensitive conductor 1 in the charge-stripping device 7 after the separation unit 6. Transfer ghosts therefore arise, caused by the opposite-polarity electric charge remaining on the photosensitive conductor 1 surface, or remaining as spatial charge in the photosensitive conductor 1 interior.
Thus under the circumstances, when the photosensitive conductor 1 has received a succeeding main charge through the main-charging unit 2, the opposite-polarity charge remaining after irradiation by the charge-stripping beam negates the charge from the main-charging, which lowers the superficial electric potential on the photosensitive conductor 1. Herein, in reverse development, the portions that are exposed, by which the superficial electric potential is lowered, are developed with toner. When the portions in which the superficial electric potential is lowered by a transfer ghost as described earlier are exposed, however, their superficial electric potential is lowered further, generating a difference in electric potential between them and the other exposed portions, such that they are developed with toner in excess of normal. Density irregularities, wherein the image density in these portions is thickened, therefore arise. The density irregularities are especially pronounced in halftone images and images that are halftone reproductions.
Making the transfer voltage smaller than the superficial electric potential on the photosensitive conductor 1 would do away with the problem of transfer ghosts, but in that case, the toner image would not be transferred to the transfer medium. This is because when the transfer voltage is applied through the transfer medium 8, the transfer voltage is shielded by the transfer medium 8, which remarkably diminishes the applied voltage that acts on the photosensitive conductor 1 surface.
Another solution means would be the method of suppressing the voltage applied directly to the photosensitive conductor by increasing the transfer voltage in stages at the start of transfer voltage application. Nevertheless, with this method, the other stages of the transfer voltage have to be controlled, which therefore not only complicates the apparatus but also raises its cost.
Still another means would be the method of enlarging the width of the main-charging device and meanwhile increasing its output. This is designed, in other words, to uniform the main charge by applying the large main-charging voltage to the photosensitive conductor 1 surface over a longer term. Nevertheless, with this means the largeness of the main-charging device ends up enlarging the apparatus overall. In addition, increasing the output of the main-charging device generates large amounts of ozone and nitrogen-oxide discharge products, leading to the problem that this deteriorates the photosensitive conductor surface.